The invention relates to a magnetic field detecting circuit using a Josephson device applied for a highly sensitive magnetic sensor, current meter, displacement detector, high-frequency signal amplifier and the like.
FIG. 2 is a circuit diagram of SQUID (RO-SQUID) where a resistor 2 and a coil 3 connected in series with each other are connected in parallel with a DC-SQUID 7. A magnetic field signal from a input coil 4 enters the DC-SQUID 7 to be converted into a critical current value, an oscillating frequency is shifted, and then a magnetic field is detected.
However, because the conventional RO-SQUID has the DC-SQUID 7 incorporated directly into the oscillator circuit a practical design must consider both the characteristics of oscillating condition of the RO-SQUID and a sensitivity of the DC-SQUID. This requires a complicated design and results in unsatisfactory sensitivity. A large inductance of the DC-SQUID arises in the above case increasing the difficulty of oscillation control from an external control line. Thus series connection for realizing the multi-channelization is impossible, and each channel must be connected independently for a plurality of channels. A problem arises because of the increases of the number of signal lines, resulting in cross talk between signal lines, and difficulty in carrying out accurate measurement.